I narrowed it down to one line that crashes the compiler:
actual is a struct passed by reference to the function, data is also a struct.
actual.score = Score(actual, data);
I've checked to make sure that score is a member, and it is.
The Score function's declaration looks like:
and the declaration of the function that this line is in is:
double Score(AEE addeff, datavar data);
The AEE struct is:
void Create_data(AEE &actual, datavar &data, total_net &net, double fake_st_dev);
double score, mean, st_dev;